Yet they are seldom seen. Analysts wonder why.

Bruce Bartlett, of the National Center for Policy Analysis, suggests in a recent paper that the disappearance of the bills may be due to the rise of the underground economy -- in which participants attempt to shield financial transactions from Internal Revenue Service scrutiny. Transactions within this sector surged sharply following substantial increases in marginal income tax rates under the administrations of presidents Bush and Clinton.

• Estimated unreported income has been increasing since the late 1980s -- when Reagan-era tax reforms started to unravel.

• Businesses are 10 times more likely than citizens not to report a dollar of income -- possibly because it is more likely to be in cash -- and farmers are 15 times more likely to shield revenue from the IRS.

• As of last year, the amount of U.S. currency in circulation totaled $458 billion -- with $292 billion of that being in $100 bills.

• Over half the total of all U.S. currency is held abroad -- with $100 bills being particularly popular.

Recently, over 80 percent of any increase in U.S. currency has immediately flown out of the U.S.

Source: Peter Brimelow, "Going Underground," Forbes, September 21, 1998.